Designers, following Moore's law, continue in their attempts to shrink the size of transistors. As transistors become smaller and smaller, gate dielectric layers have also become thinner and thinner. The continued decrease in the thickness of gate dielectric layers is leading to technical problems. Leakage through a silicon dioxide dielectric layer of a gate increases exponentially as its thickness decreases. Gate dimensions that are proposed for the future will require dielectric layers that are so thin they may stray from purely “on” and “off” states. Instead, leakage may lead to a low power, or “leaky”, off state. This challenge must be addressed for the success of future transistor generations.
One alternative that is being proposed is to use high k materials in place of silicon dioxide as the gate dielectric layer. High k refers to a high dielectric constant, a measure of the ability of a material to resist the formation of an electric field within it. Differing materials possess differing dielectric constants. High k materials include compounds of oxygen such as hafnium dioxide (HfO2), zirconium dioxide (ZrO2), and titanium dioxide (TiO2), among others, and possess a dielectric constant above 3.9, the value of silicon dioxide.
However, the use of materials other than silicon dioxide as a dielectric material affects other components in the transistor structure. For example, electrodes have commonly been fabricated of doped polysilicon for use with silicon dioxide dielectric stacks. However, it has been found that doped polysilicon does not perform well with high k dielectric materials. When matched, for example, with a hafnium dioxide material in the gate dielectric stack, a doped polysilicon electrode suffers from a poor work function.
It has thus been proposed to use materials other than doped polysilicon for use as a gate electrode with high k dielectric materials. One such class of materials proposed for use as an electrode are conducting metal oxides. However, the use of conducting metal oxide as an electrode material presents several new challenges. First, the selected material must demonstrate stability over a range of temperatures. Virtually all the existing conducting metal oxides have been found inadequate for use as an electrode in combination with high k dielectrics because they have degraded with anneals. Conducting metal oxide materials have tended to dissociate, sublime, and degrade at high temperatures. This can result in changes in their electrical characteristics or the structure of the dielectric films underneath them which leads to a failure of electrical performance of the device. Additionally, a conducting useful metal oxide material must provide an acceptable work function. However, the desired work function can vary depending on whether a p-MOS device electrode or n-MOS device electrode is desired. The former generally need a high work function value, and the latter a low work function value.
Accordingly, it is desirable to identify new materials and methods of applying these materials for use as p-MOS device electrodes with high k dielectric layers. The desired process and materials should provide an effective high work function in conjunction with high k materials in gate dielectric layers. It is also desired to develop a p-MOS device electrode which performs well over a range of temperatures. In addition, it is desirable to develop these materials and methods so that they are suitable for use with current processing techniques used in integrated circuit fabrication. The present invention addresses one or more of these needs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.